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Sensitometry

In this chapter, you'll find some formulas relative to sensitometry.
They are given in a pretty rough mode, the page will be enriched further.

Meanwhile, you can check the following sites for a more verbal explanations:
In French, at Christian Judei site
In English: a Kodak document

Sensitometry is the art of measuring the relation between an exposure and its effect on a sensitive surface.
Here is an example for a B/W negative surface (film or paper):

A positive has a falling slope while a color surface is composed of three curves, one each basic color.

Units

Light flux: 1 lux = 1 candela/m2 (1 foot-candle = 10.764 lux)
Exposure: E = lux . seconds      often indicated by its logarithm: log(l.t)

Density

Paper:

Film:

Density, mainly with papers, is often indicated in percentage. The conversion formula is the following:
or

Film Speed

Film speed is measured for the quantity of light producing a density of 0.1 over the film fog and base density.

Paper Speed

Paper speed is measured for the quantity of light producing a density of 0.6 over the paper fog and base density.

Paper contrast

The paper contrast is given according two points:
Hs = exposure to obtain a density of 0.04 over the paper fog and base density.
Ht = exposure to obtain a density of 90% of the maximum density obtainable plus the paper fog and base density.

Light Meters

The light meters are set to obtain an average gray around a 18% reflectance.